Power consumption is typically critical for all mobile devices, among them wireless communication devices. In order to reduce the power consumption, the mobile device can be set to a minimal operating mode, called “idle” mode or “sleep” mode, when the device is not used for voice or data communications. In this minimal operating mode, only a minimal logic is powered and clocked, and most of the components or functions of the device are disabled. During this minimal operating mode, the device checks with (or listens to) a current base station, infrequently and for a short period of time, for pending incoming traffic or for a wake-up order from the current base station. When receiving the wake-up order, some components or functions of the device are switched to an active operating mode, also called “active” mode or “wake-up” mode.
For instance, for a device using WiMAX (Worldwide Interoperability for Microwave Access) technology, when the device is in the idle mode, the device listens to the current base station during a paging listening interval every paging cycle, a paging listening interval lasting usually between two to five frames, preferably two frames. In WiMAX technology, one frame duration is equal to 5 milliseconds (5 ms), and one paging cycle duration (or paging interval) may be equal to 1 second (or 1.5 seconds) or 200 frames, of course the paging cycle duration can be set by the network. Outside of the paging listening interval, the device can shut-down most of its components (or functions) to reduce the power consumption. During the paging listening interval, the network can send a signal to the device to switch the device (or the components or the functions) to the active operating mode in order to handle an incoming call for example. As the device knows the next paging listening interval date, the device can shut-down as many components or functions as possible for the longest possible duration, in order to minimize power consumption. Nevertheless, in order to wake-up some components or functions of the device at a proper time, some minimal logic chip of a base-band chip of the device are kept powered-up and clocked by a low power clock. This low power clock is generally a low precision clock and presents frequency error which may affect the date (or time) to wake-up the components (or functions) of the device. The frequency error is due to several factors like a change in temperature, a change in input current voltage, the aging of the low power clock.
To compensate the frequency error of the low power clock, the U.S. Pat. No. 6,873,215 “Cellular phone terminal and intermittent reception control method to be used in cellular phone terminal”, which is incorporated by reference, presents a system using the periods of activity of the device to perform the calibration of the low power clock, by using a local high frequency clock and the low power clock, and by comparing the number of pulses of the low power clock to a theoretical expected number. But this method is not acceptable for the precision requirements of a 4G system for example. As shown in FIG. 1, a controller CTRL receives both a high frequency clock HFC and the low power clock LPC. This controller can turn on or off the high frequency clock HFC and other devices (not shown). The controller also receives environmental information to detect temperature and battery level changes. To calibrate the low power clock, the high frequency clock and the low power clock are running in parallel from a common start date corresponding to the occurrence of a rising edge of the low power clock pulse. The system counts the number of pulses of the high frequency clock and the low power clock, up to another low power clock pulse edge. Because the high frequency clock is asynchronous to the low power clock, the system only starts to count the number of pulses of the high frequency clock on a first pulse edge of the high frequency clock from the start date, as shown in FIGS. 2a and 2b. The time interval duration D between the start date and the end of the counting is estimated from the number NF×TF, where NF is the number of pulses of the high frequency clock and TF is the duration of a pulse of the high frequency clock. As the clocks are asynchronous, two pulses of the high frequency clock can be missed up (at the start and at the end of the counting). Therefore, if ε is the error on the estimation of the counting time interval duration, the relative error on the estimation of the low power clock frequency is
      ɛ                  N        F            ⁢              T        F              ,which can be up to
            2      ⁢              T        F                            N        F            ⁢              T        F              =            2              N        F              .  This can be used to derive how long the calibration needs to be run to have a given accuracy. For example, in a WiBro (Wireless Broadband) based communication system where the clock of the base station runs at 10 MHz, for a 1 ppm (one part per million) accuracy, at least 2 millions cycles of the fast clocks are needed. This takes 0.2 seconds, which is much more than the paging duration for WiMAX, which ranges from 10 to 25 milliseconds. On many paging intervals, the average of the ε error will be about TF. However, this will require many paging intervals, which are separated by 1 to 1.5 seconds typically. Getting the error low is incompatible with a quick calibration, which is necessary when a temperature change is detected for example. The on-going calibration is not compatible with the precision requirement of a 4G system such as WiMAX. Long calibration periods would be needed with these implementations.
Furthermore, it may be necessary to make provision of waking-up the components (or the functions) of the device gradually. For example, a VCTCXO (Voltage Controlled Temperature Compensated Crystal Oscillator) or a TCXO (Temperature Compensated Crystal Oscillator) high frequency clock may need up to several milliseconds to be stabilized, and may need to be woken-up ahead of time to be fully operational when a next paging cycle starts. The existing solutions typically require computing wake-up durations. But as the calibration of the low power clock may not be accurate, an additional error may be introduced in the wake-up date.